Alfisol, Item 007: IExFit, version 3.1
Fit adsorption data based on ion exchange reaction models.
Display solid & liquid & gas phase speciation diagrams as a funciton of pH.

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Add Gases

In the upper left corner here (see figure below), you decide if gases exist or are allowed to exist. If so, you also decide which ones and how much is initially present in the headspace per liter of liquid in your experiment.

The choices are:
(1) infinte volume (total pressure is fixed): select this to mimic a liquid with the whole atmosphere above it.

Examples include raindrops, dew, rivers, and oceans.

(2) initial volume is fixed (total pressure is variable): select this to mimic a closed bottle with a limited headspace above the liquid. That is, the bottle is closed with a cap.

Examples include a soda bottle, and a laboratory container or bottle.

(3) initial volume is variable (total pressure is fixed): select this to mimic a closed bottle with a variable headspace above the liquid. That is, the bottle is closed with an air bag (or balloon) at its mouth where a cap would normally be. If you are using a balloon, then presumably the balloon is large enough so as to never be filled to the point where it starts adding pressure when more gases enter the balloon. That is, the balloon is always limp, and the pressure inside the balloon equals the atmospheric pressure outside the balloon. The purpose of the balloon (or air bag) is only to avoid the internal air from mixing with the external air.

Examples include bottles with expanding or shrinking chambers that maintain a constant pressure.
It could perhaps also include something like the Biosphere 2 project, whose internal pressure can be controlled by expanding chambers.
And most importantly, it is intended to approximate soil air environments if we assume that there is a fast equilibria of gas exchange at the liquid-soil air boundary, that the soil air does not mix with the above ground atmosphere (or its diffusion outward is very slow), and that the pressure below ground is kept constant and nearly equal to the above ground atmosphere. That is, this mimics soil air if the the soil matrix is acting like a large inflatable bag of trapped air around the vicinity of the soil water.

You can also tell IExFit that your exercise involves a curved gas-liquid boundary. Note that the published Henry's Law constants (KH) assume that the boundary is flat. However, it changes if the boundary is curved according to the Kelvin Equation. The last figure below shows how you can change the parameters used by the Kelvin Equation in the "pKH Curvature Parameters" window, which opens when you click on the "edit parameters" link.

The important parameter of interest is the radius of curvature. Also, is it a concave or convex curvature. It is convex if its an aerosol (e.g., raindrop or mist) or a drop on a hydrophobic surface (e.g., some soils following a fire event). It is concave if it is in contact with a hydrophilic surface, as are most soils.

In the middle section of this "Add Gases" window, you select which gases are allowed to be present in the headspace. You also enter the initial percent of each gas in the headspace. The total percent must add up to 100%, and the remaining amount is considered to be an inert gas. Note that H2O(g) is always present at equilibria in closed containers at 100% relative humidity, but it can start out with a lower value as its initial condition.